Prove You Wrong
by JustMe133
Summary: What starts as a normal Friday night for Casey and Derek turns into so much more for the both of them. Rated M to be safe. DASEY. If you don't like "step-sibling-cest", don't read this. ONESHOT.


**If you follow me on Tumblr, you shouldn't be surprised. If you don't, you should know that I've recently been reunited with this show and thus, the pairing. Hence, the story.**

**No, sadly, I don't own Life With Derek or the characters. No copyright intended.**

…

Regular – story  
_Italics – Remembering/Flashback  
__**Bold italics – thoughts**_

…

Casey wasn't sure how this had happened. It had been a normal Friday night in their house. She definitely didn't plan for it to end like this.

_Casey and Derek sat on the couch, listening to her mom and his dad tell them what was planned for the night._

"_Now, George and I are going on our date night. We'll be back by 11. Marti will be at the sitter's, and Lizzie and Edwin will be at their friend's houses," Nora was saying. Casey was paying rapid attention while Derek was blindly flipping through the TV channels in his armchair._

"_Derek," George told him, making the teen just grunt in acknowledgement, "You're going to be home alone with Casey. Please don't kill each other."_

"_Casey, please."_

"_I'm not the one you need to worry about mom," Casey said, shooting a disdainful glare at Derek who just smirked._

"_As long as Klutzilla doesn't bother me, we won't have a problem," he retorted, eyes never leaving the TV. Casey grit her teeth and gave her mom a forced smile._

"_We'll be fine. You guys go have fun okay?"_

"_Okay," Nora said, giving her daughter a hug. George ruffled Derek's hair, making him swipe at him and miss. Casey hugged her younger sister and stepsiblings before they were gone, leaving Casey and her annoying stepbrother alone._

Okay, so it wasn't that bad; they had been left alone before, right? Well sure, but then _this_ happened. Casey rolled onto her side and caught sight of the clock next to it. She only had about 30 minutes to get a grip on this before her mom and stepdad returned.

_She had been on the couch, curled up with a book. He had been in his armchair, TV blaring out a hockey game. She ignored it as much as she could before she had spoken up._

"_Der-ek! Will you turn that down!" He looked at her before he turned it up even louder. She grimaced and threw her book down before she stood up and made her way right in front of him, hands on her hips._

"_Move out of the way Space Case," he said, trying to look around her. She made a grab for the remote, surprising him. They both began to tug on it before he pushed her down; she hadn't let go though, so the two of them had tumbled to the ground, him on top of her. "Let go."_

"_Not until you turn it down!"_

"_No!"_

"_Der-ek!" she had exclaimed, lifting her hips a bit as she struggled for it. Derek had stopped moving when she had done this, his eyes darkened with widened pupils. She saw this and stopped struggling. "Derek? You okay?" He had just stared at her then, eyes locked on hers._

She took a shuddering breath as she remembered what happened next. It had all been such a blur though; fast, rough, and definitely not how she had originally expected something like this to be. Slowly, she began to wiggle to the edge of the bed as she remembered it, her face heating up.

_He had tossed the remote away, his eyes still locked on hers. Then, he had pinned her arms above her head and pressed his crotch down on hers, pinning her down on the living room floor._

"_Der-ek! Let me go!" Derek was still silent as he watched her before he pressed his lips to hers. She stilled under his body as his mouth worked against hers. Against her better judgment, she kissed him back with the same urgency that he had. Within seconds, their tongues were battling for dominance before he pulled away and picked her up. "Derek!" She squealed, holding onto him tightly. He pressed his lips to her neck as he made his way up the stairs, her legs wrapped tightly around his waist and her hands tangled in his hair._

_Soon, they were in his room. He had tossed her on the bed before he joined her, their lips meeting again as they both made fast work of their clothes. Soon, there were only his boxers and her panties stopping them from continuing any farther. They were both panting hard, and she knew he could see the confusion in her eyes. _

"_Do you want to do this Case?" he had asked her, voice rough as he stared at her, eyes filled with something she had never seen him look at her with._

_**Oh God,**_ Casey thought, rubbing her palms against her tired eyes as she thought about this. _**I wish this was a dream but it obviously wasn't… What do I do now?**_

"_Yes," Casey whispered, surprised with herself. He had practically given her an out, and she still agreed to it. His hands made fast work of the last two articles of clothing before he cupped her face._

"_Is this…"_

"_Yeah," Casey said, eyes closing._

"_It's going to hurt," he said, kissing along her jaw._

"_I know," is all she said as he began to push into her. Her hands were braced on his shoulders and her eyes were closed. He pressed a kiss to her lips as he went all the way in. She cried out against him, her nails digging into him, leaving marks._

"_I'm sorry Baby," he murmured. She couldn't even focus on him saying that before the pain was gone. Slowly, he began to move. After that initial moment, it really was just a blur._

Casey felt her face heat up even more and her breath hitch a bit. She really had to get out of this bed and into her own.

"Where ya goin'?" A tired voice asked as she swung her legs over the side of the bed. She blushed as she looked over her shoulder to see a sleepy Derek watching her.

"Mom and George will be back soon… I need to get to my room," she said softly, turning to look away from him. Derek was silent for a moment before he stood up and rummaged through something.

"Here," he said, holding out a shirt to her; it was one of his, and way too big for her slighter frame. "I think I tore some buttons off yours, so you can borrow mine." She was surprised by this but took it from him.

"Thanks," she said, slipping it on over her naked body. She scanned the room and found her panties hanging off his headboard. Blushing, she grabbed those and slipped them on under the shirt. She couldn't look at the headboard without remembering how much noise they had made.

"_God, Derek," she had moaned out, her legs wrapped around him and her nails scratching down him. Derek moaned out in return as he pushed in harder, successfully making his headboard bang loudly against the wall._

Casey shook her head and made her way to his door to leave. Before she could, a hand was on her shoulder.

"Case."

"Yes?" she asked, not looking at him. He frowned and let go of her.

"Goodnight."

"Goodnight Derek." As Casey left his room and went to hers, she couldn't help thinking this was a mistake, but there was nothing she could do about it now.

_**God, I had sex with my stepbrother**_**.**

…

The next morning, as Casey was making her way to the bathroom to go shower, she ran into a very wet Derek with nothing but a towel around his waist.

"Geez Derek, can't you at least _move_?" she asked, trying to get past him. He rolled his eyes and walked past her, bumping her shoulder with his.

"You know you like it Space Case." She growled and stormed into the bathroom.

_**Looks like having wild sex doesn't change things, **_she thought as she began to strip for the shower. She stopped though as she caught sight of her neck.

"DER-EK!" she screamed, hearing him chuckle from the doorway; he was rubbing his hair with his towel, and he was now dressed in boxers and a shirt.

"You rang?"

"Look at this!" she hissed at him, bringing him into the bathroom so they could talk. "Look at my neck!" He titled his head and inspected it.

"Woo," he said, letting out whistle, "I did a pretty good job didn't I? Although you didn't too bad of a job yourself," he said, lifting his chin. Casey's eyes widened at the number of dark, red hickeys that covered his neck.

"Wow. I-I did that?"

"Yep," he said, "and this," he said, pulling his shirt up and over his head, turning his back on her. Her hands flew to her mouth as she studied the scratches that ran across his shoulders and all the way down his back.

"I'm sor-"

"Don't be," he said, cutting her off and pulling his shirt on, smirking at her. "If I knew you were this wild in bed I would've given in to those urges a long time ago."

"Der-ek!" He chuckled and tossed his towel in a hamper.

"You wanna do it again dontcha?" he asked, making her roll her eyes as she started the shower water.

"No," she said, shooting him what she hoped was a glare. She caught his face flicker to surprise before his smirk returned.

"Oh really? And why is that?" Casey was silent before she sighed and looked at him.

"Because I don't want to just be a fuck to you Derek. Now get out so I can shower." She pushed him out before he could see the first tears fall. Derek stood in the hallway, just staring at the bathroom door, unsure what to do now.

_**Surely she knows she's not just another girl to me, doesn't she?**_ He thought, shaking his head. Looked like he was going to have to work on that.

…

At breakfast that morning, Casey was wearing a decorative scarf to hide her neck while Derek was proudly displaying his. She didn't look at him as she began to speak to her mom.

"I'm going shopping with Emily today. Can I borrow your car mom?" she asked, spooning some cereal into her mouth.

"I can drive you if you want," Derek said, making Casey choke lightly on her cereal while everyone else stared at him. "What?"

"Why?" she asked him, not believing her ears.

"What, I can't do something nice for you?"

"_**NO**_."

"Do you want a ride or not?" She grit her teeth and sighed.

"Fine." He smirked and she just looked away from him before she stood up. "Whenever you're ready, then we can go," she said stiffly, moving to the door.

"Great," he said, getting and following her. Their family watched as the two stepsiblings walked out, Casey looking agitated and Derek looking pleased.

"Anyone else feel like they missed something?" Nora asked, getting one loud collective "Yes!" from everyone.

…

Casey looked at Derek; the two of them were sitting in his car, waiting for Emily to finish getting ready.

"Why are you doing this?" she asked, sounding worn out. "Wasn't last night enough for you?"

"Nope," he said, smirking at her.

"Well you're not getting any more from _me_," she said, crossing her arms over her chest.

"Why not?"

"Because I wanted to wait till I was with the right person, and in love, and now… that's all gone. In case you forgot, that was my first time."

"It was mine too."

"What?!" Casey asked, looking at Derek, who was staring out the windshield.

"Yeah. And I'm glad it was with you Case." Casey blushed and began to play with the end of her scarf.

"_Well…_ I'm glad my first was with you and not some creep who would regret it."

"Do you regret it?"

"Derek," she sighed, looking at him. "I believe in love. You don't. That should be enough to answer your question."

Derek was silent then as Emily got out to the car. As the two girls began to sing along with whatever music was playing, he was thinking.

_**Does Casey love me? Is that what she was getting at? **_He glanced at her as he thought about that. _**She really is beautiful. And smart. And stubborn. But that's what makes our arguments so great… I can get under her skin like no one else can and she can do the same to me. And of course I've wanted to sleep with her since I realized I'd be living in the room right next to a total babe… And then last night, it finally happened. But would I want to call her my **__**girlfriend**__**? **_Derek thought about this as they pulled up into the mall parking lot and he got out to follow the two of them in. _**I mean, sure we get along great when we're not fighting or working together, and she's great to cuddle with, but she's unlike any girl I've ever been with. Maybe that's what I've needed all along though. Maybe… I've always needed Casey. How will dad and Nora take it? I mean, they never legally adopted the other's kids, so we could… it's not against the law, and not really incest since we're not blood related. But, the question is, **__**do I love Casey McDonald?**_

Derek felt a grin come to his lips as he watched Casey laugh and smile with Emily as the two of them looked through the clothes. Derek found himself pulled into a chair on the opposite side of the dressing rooms as the two of them began to try on clothes; every few minutes one or the other would come out, showing him their outfits. He felt his heart leap into his throat as Casey came out in a very tight fitting, low necked purple dress. She saw his stare and smiled.

"What do you think?"

"You look…_great_," he said softly, making her blush.

"Thanks," she said, disappearing back into the dressing room. He heard a thump in the seat next to him to find a young man next to him.

"Your girlfriend drag you shopping too?" he asked, making Derek think about that. _**Girlfriend… Casey…Girlfriend…Casey…Is my girlfriend. Casey McDonald is my, Derek Venturi's, girlfriend. **_The thought of those words together made him smile.

"Yeah. Yours?"

"Yeah," he said, nodding towards a pretty redhead girl who disappeared into the dressing room next to Emily's, who just came out, Casey soon behind her.

"Which one is yours?"

"The one with the long brown hair," he said, looking at Casey as her and Emily looked at perfumes.

"It's nice being in love isn't it?" The guy asked, staring at his girlfriend the way Derek knew he was looking at Casey.

"Yeah, it is."

_**Damn, I'm in love with her.**_

…

"I'm getting hungry," Emily said as the trio made their way around the mall, Casey and her loaded down with bags.

"Yeah, me too," Casey murmured as the two of them headed off to the food court. She stopped when she realized Derek wasn't with them. "Are you joining us?" she asked him, making him shake his head.

"No… I've got somewhere to be first. I'll be right back." Casey watched him turn and hurry away before she rolled her eyes.

_**Whatever, not like I wanted to spend time with you or anything**_.

"It's weird," Emily said as the two of them got in line for food.

"What is?"

"The way Derek's acting."

"Oh? How is it weird?"

"Well… usually he'd be bragging about whatever girl he's with, and he hasn't done that once. He's been unusually quiet." Casey looked down at her hands as her friend continued to speak. "Do you know who gave him all those hickeys anyway?" Casey shook her head as she quickly ordered her food. "Case? You okay?"

"Yeah," she said, giving her a smile. "I just don't like talking about Derek and his love-life. I really couldn't care who he's sleeping with this week," she said, her voice catching. Emily saw this and frowned.

"You sure you're okay?" Casey was silent as the two of them settled down to eat, her mind running.

_**Maybe I should tell her what happened between us. I mean, she is my best friend… but she has a big mouth too… But I really want to tell someone…**_

"Em, if I tell you a HUGE secret, will you promise not to tell anyone?" she asked suddenly, making her friend perk up.

"Of course!" Casey opened her mouth but no words would come; instead, she tugged down her scarf a bit to reveal her own hickey-covered neck. Emily stared at Casey for a moment before her mouth fell open. "OH MY G-"

Casey quickly covered her mouth up with her hand and spoke quickly. "It just happened," she began, wanting to get the explanation over with. "We were fighting over the remote like always and then he's kissing me, and then we're in his room. That's all the details you get. Now, if I remove my hand, will you promise not to scream?" Emily nodded and Casey removed her hand.

"Wow…" she began, looking at her best friend, who looked like a kicked dog. "This bothers you doesn't it?"

"A bit… I can't believe I let Derek get to me like that," she muttered, resting her head on her hand.

"Well… I think you two would make a cute couple."

"Ew Em, no. He's my stepbrother."

"So? You two aren't blood-related…"

"Em-"

"And that would explain why he was acting so different today-"

"I told him I didn't want to be just another fuck to him. He's just trying to stay on my good side so we'll do that again, which won't happen."

"Really?"

"_**Yes**_."

"I don't know Case… I've lived across from Derek for a _long_ time…"

"And I've lived _with_ Derek. I doubt one night of sex will change him." Emily shrugged and smiled at her. "What?"

"Nothing." Casey rolled her eyes and rubbed her arms.

"It's cold in here," she muttered, feeling a heavy warm presence engulf her shoulders; looking, she found Derek's leather jacket over her and him moving into the seat next to her.

"Ladies," he said, grinning at them, eyes trained on Emily. "She told you." Emily's mouth opened in surprise and Casey buried her face in her hands. "I don't care."

"How did you know?"

"I can read you like a book Em." Emily blushed now and he laughed, swinging his arm around Casey's shoulders. "Anyway, Case," he said, pulling a box out of his jacket, where it still hung on Casey's shoulders, "I got you this." Casey looked at him and he pushed the box to her. "Well open it." She took the box and just stared at him again before she lifted the lid.

"Wow," she breathed out, looking at the necklace that hung in box. Gently, she picked it up and inspected it. It was a simple silver heart with a light decorative border etched in it; it hung on beautiful silver box-chain. "It's beautiful."

"Read the back of it," he said, his arm still on her shoulders; she looked at him and he smiled. "Go on." She turned it over and felt tears well up in her eyes.

"To Casey, Love Derek," she whispered, looking at him. Emily's eyes were watering too as Casey looked at it still. "Derek…" she said, shaking her head before putting it back down in the box and closing it. She gave him back his jacket and walked away. "I'll be right back," she murmured, hurrying away. Emily and Derek watched her, Derek looking put down.

"She's a huge romantic. I thought… I thought she'd like it."

"She does," Emily said, giving a sad smile. "She just doesn't like being wrong. I'll go talk to her." Derek watched her go before he pulled his jacket back on and put the box into the pocket.

…

"Casey-"

"Oh God, what have I done?" she whispered, pacing in the bathroom. "He… he loves me."

"And you love him."

"No I don't."

"You so do Casey," Emily said, placing her hands on her best friend's shoulders. "And you know how I know?"

"How?"

"Because I know you. You might not want to love him, but you do. You're gonna have to lose this fight." Casey was shaking her head and then sighed. "You wouldn't have slept with him if you didn't feel _something_." Casey bit her lip as her face began to heat up. "Admit it Casey. You love Derek Venturi."

"When his dad and my mom kick us out, we're moving in with you."

"I'll share my bed with you; he can sleep on the floor." Casey giggled and hugged Emily.

"Thanks Em. You're the best friend I've could've asked for."

"I know," Emily said, pushing her. "Now go tell Derek you love him." Casey blushed and made her way out of the bathroom.

_**I love him. I'm in love with Derek. How did this happen? Yesterday he was just my annoying stepbrother, and now… he's my annoying stepbrother that I'm in love with.**_

"Casey."

"Derek," she said, sitting next to him, "do you really l-"

"Yeah Space Case, I do. And trust me, I never meant to fall in love with a grade-grubber Klutzilla like yourself-"

"Der-ek you jerk!" she exclaimed, hitting him in the shoulder, making him smirk as he caught her hand.

"But I did," he finished, smiling softly. "I love you Casey. And… well…" _**What the hell? I'm at a loss for words? When has that ever happened?!**_ "Typically, when two people are in love… they date…"

"What, is the great Derek Venturi asking me out?" Derek had the decency to look embarrassed, making her smile. "And yes."

"Really?"

"Don't sound so surprised. I love you too you know."

"You do?" Casey nodded as he pulled her to him and kissed her. As they pulled away, Derek grinned.

"So does this mean we can keep having sex?"

"DER-EK!"

...

**Yay, my first LWD oneshot is complete. Thanks for reading!**

**-JustMe133**


End file.
